Process for making improved phosphate phosphors from monetite

ABSTRACT

IMPROVED PHOSPHORS ARE MADE BY CONTROLLED HEATING OF ANHYDROUS CALCIUM HYDROGEN PHOSPHATE PARTICLES, THE PARTICLES BEING MAINTAINED IN AGGLOMERATION-PREVENTING MOTION THROUGHOUT THE HEATING PROCESS, TO FORM CALCIUM PYROPHOSPHATE WHICH IS THEN MIXED WITH OTHER PHOSPHOR RAW MATERIALS AND FIRED TO FORM THE PHOSPHOR.

United States Patent 3 654 173 PROCESS FOR MAKING IMPROVED PHOSPHATE PHOSPHORS FROM MONETITE Martha J. B. Thomas, Winchester, Ernest A. Dale, I-Iamrlton, and Keith H. Butler, Marblehead, Mass, asslgnors to GTE Sylvania Incorporated No Drawing. Continuation-impart of apphcatiou Ser. No. 819,949, Apr. 28, 1969. This application Apr. 14, 1971, Ser. No. 134,027

Int. Cl. C09k 1/36 U.S. Cl. 252-3014 P 4 Claims ABSTRACT OF THE DISCLOSURE ilmproved phosphors are made by controlled heating of anhydrous calcium hydrogen phosphate particles, the particles being maintained in agglomeration-preventing motion throughout the heating process, to form calcium pyrophosphate which is then mixed with other phosphor raw materials and tired to form the phosphor.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of Ser. No. 819,949, filed on Apr. 28, 1969, now abandoned.

In a copending application, filed on even date herewith by E. A. Dale and M. J. B. Thomas, entitled Process For Making Improved Halophosphate Phosphors From Brushite, a process is disclosed for preparing phosphors from CaHPO -2H O. The instant application relates to a process for preparing phosphors from CaHPO BACKGROUND OF THE INVENTION Field of the invention This invention relates to phosphors containing calcium and phosphate ions, such as pyro-, orthoand halophosphate phosphors. It particularly relates to a process for making phosphors having improved luminescent properties.

Description of the prior art CaHPO (monetite) is a material commonly used in the preparation of many ditferent phosphors. In the manufacture of CaHPO its hydrated analog CaHPO -2H O is first precipitated under controlled conditions to obtain a desired particle size from a solution containing phosphate ions and calcium ions, such as, for example, from a solution of (NH HPO and CaCl Such a process is disclosed in US. Pat. 3,505,012, issued on Apr. 7, 1970 to E. A. Dale et a1. and entitled Process For Making the Brushite Form of Calcium Hydrogen Phosphate. The precipitate is then dehydrated by controlled heating in the mother liquor to a temperature between about 60 and 95 C. The CaHPO thus produced was then washed, dried and could then be directly used in the preparation of phosphors.

In the preparation of such phosphors, CaHPO, was mixed with other raw materials, such as CaCO CaCl CaF and with activating compounds such as Sb O The mixing was then followed by controlled firing to yield luminescent materials.

During the firing process the following intermediate reaction would occur:

The weight of water produced was 6.62% of the weight of Cal-IP0 The evolution of this water vapor could easily strip out some of the other constituents in the phosphor blend, such as chloride (in the form of HCl), phosphorus (in the form of POCl), antimony (in the form of SbCl ice or Sb O and the like, which, in turn, could upset the desired stoichiometry of the phosphor.

If the CaHPO were converted to Ca P O such as by firing in a crucible, before being blended into the phos phor mix, many of these problems could be eliminated. However, evolution of the water through the cake of CaHPO could result in sintering and agglomeration of the particles, the elfect of which would be to increase the average particle size of the resultant CfizPgOq- This, in turn, would reduce the efliciency and maintenance of phosphors produced therefrom, since those properties are highly dependent on the particle size of the phosphor.

Another disadvantage to the use of CaHPO, in the phosphor blend is that corrosive gases, resulting from the evolution of water and its reaction with other phosphor constituents, can be liberated into the controlled atmosphere of the phosphor furnace.

SUMMARY OF THE INVENTION This invention discloses a process for producing Ca P O from CaHPO, wherein the particle size and shape and the crystalline morphology of the resultant Ca P O are substantially unchanged from that of the CaHPO The CaH-PO, employed in our process may be that disclosed above in 3,505,012, wherein brushite having a desired purity, reactivity and particle size is produced and then converted to the anhydrous monetite (CaHPO by controlled heating of a slurry thereof.

The CaHP O thus produced is then dried by, for example, centrifuging and then baking at 325 F. In the prior art this anhydrous CaHPO, would then be used as a starting material for phosphor synthesis by blending with other raw materials, as previously mentioned. However, we have discovered that if the Cal-IP0 particles are converted to Ca P O before being blended with other raw materials, an improved phosphor will be produced. The conversion must be effected under such conditions that the particle size and shape and the crystalline morphology of the Ca P O are substantially unchanged from that of the CaHPO The conversion is effected by heating the CaHPO particles to a temperature that will produce 'yCa P O-;,

while the particles are maintained, throughout the heating process, in a motion that separates the particles from one another. The motion must be sufiicient to prevent substantially any agglomeration or sintering together of the particles throughout the conversion process. That is to say, the motion must be sutficient to prevent any clustering together of particles that would result in an increased particle size when determined by, for example, sedimentation analysis or a Fisher Sub Sieve Sizer. The heating temperature should be in the range of about 420 C. to 750 C., the lower temperature being that necessary to attain substantially complete conversion. Above about 750 C., /8-Ca P O will be formed, instead of the desired 'YCEI2P2O7; 'yCa P O is preferred since it has higher reactivity in the solid state than the [3 form and results in phosphors having better luminescent properties than phosphors prepared from }9Ca P O DESCRIPTION OF THE PREFERRED EMBODIMENT A precipitate of CaHPO 2H O was prepared as disclosed in 3,505,012 and was then converted to Cal-IP0 by heating the slurry as mentioned in the patent. The CaHPO, was then centrifuged and dried at 325 F. The dried CaHPO, was then charged into one end of a stainless steel, rotary, gas heated kiln, the kiln having a diameter of 6 /2 inches and a length of 7 feet. The kiln was inclined at a horizontal angle of 5 and was rotated at a speed of 8 rpm. At a kiln temperature of 540 C., the

CaHPO was converted to Ca P O in one pass, the residence time of the powder in the kiln being about 8 minutes at a charge rate of 25 pounds per hour. The motion of the individual particles of powder within the kiln was a combination of sliding, rotating and tumbling, and was sufiicient to prevent substantially any agglomeration or sintering together of the particles.

A halophosphate phosphor was made from the Ca P o so produced and was tested in fluorescent lamps. The results were compared with those of fluorescent lamps having the same halophosphate phosphor, but which was prepared from CaHPO instead of Ca P O In Test No. 1, the 082F207 lamps had an initial output of 3146 lumens versus 3115 lumens for the CaHPO lamps, a gain of 35 lumens; after 100 hours of operation, the output of each was 3051 and 3016 lumens, respectively, the gain remaining 35 lumens. In Test No. 2, the same respective gain was 6 lumens initially and 11 lumens after 100' hours. Heating means other than a rotary kiln could also be used to convert CaHPO into Ca P O in accordance with this invention. For example, the powder could be maintained in agitation in a fluidized bed while being heated to the conversion temperature. The powder charge therein and rate of gas flow through the bed would, of course, have to be such as to maintain sufiicient agitation of the powder to prevent substantially any agglomeration or sintering together of the particles.

The following are oifered as examples of pyro-, ortho-, and halo-phosphate phosphors made in accordance with this invention. In all the examples, the Ca P O was prepared from CaHPO as described above.

EXAMPLE 1 Pyrophosphate phosphor:

Raw material:

i.ss 2 7 .04 Weight in grams Ca-21 207 EH20; (NI-I HPO 10.8

Orthophosphate phosphor: Ca Zn Mg (PO zsn Raw material: Weight in grams 032F207 CaCO 5s MgCO 6.7 ZHO 17.1 S1102 3 The above materials are thoroughly mixed and fired at 2000 F. for 2 /2 hours in 8%H -92%N 4 EXAMPLE 3 Halophosphate phosphor:

Raw material:

4.ss .9( 4)a .04 Weight in grams ca P o 12,578 CaF 1,160 CaCO 3,997 Sb O 192 The materials are thoroughly mixed and fired at 2200 F. for 1 /2 hours in air. This phosphor was used in the test above.

EXAMPLE 4 Halophosphate phosphor:

4.s4 .os .ss5 .19( 4)s .os .oa9 -Raw material: Weight in grams 'Ca P O 12,578 CaCO 4,03 8 CaF 1,076 NH C1 33 5 MnCO 3 63 Sb O 43 3 CdO 254 from a solution containing calcium ions and phosphate ions; eliminating the water of hydration from the CaHPO 2H O to form anhydrous CaHPO drying the CaHPO heating the CaHPO while maintaining it in agglomeration-preventing motion to a temperature suflicient to convert it into 'yCa P O mixing the 'Y-Ca2P 07 with other phosphor raw materials; and firing the mixture to form a phosphor.

2. The process of claim 1 wherein said heating is at a temperature between about 420 C. and 750 C.

3. The process of claim 2 wherein said agglomerationpreventing motion is obtained by heating the CaHPO in an inclined, rotating kiln.

4. The process of claim 2 wherein said agglomerationprevention motion is obtained by heating the CaHPO in a fluidized bed.

References Cited UNITED STATES PATENTS 3,505,012 4/1970 Dale et al. 25230l.4 P 3,549,553 12/1970 Wachtel 25230l.4 P

ROBERT D. EDMONDS, Primary Examiner 

